Hong Zhu,Borui Xu,Yang Wang,Xiaoxia Pan,Zehua Qu,Yongfeng Mei
Hydrogels are an exciting class of materials for new and emerging robotics. For example, actuators based on hydrogels have impressive deformability and responsiveness. Studies into hydrogels with autonomous locomotive abilities, however, are limited. Existing hydrogels achieve locomotion through the application of cyclical stimuli or chemical modifications. Here, we report the fabrication of active hydrogels with an intrinsic ability to move on the surface of water without operated stimuli for up to 3.5 hours. The active hydrogels were composed of hydrophobic and hydrophilic groups and underwent a dynamic wetting process to achieve spatial and temporal control of surface tension asymmetry. Using surface tension, the homogeneous active hydrogels propelled themselves and showed controlled locomotion on water, similar to common water striders.
Controllable impact spreading behavior is critical for effective thermal management of spray cooling. However, splash and retraction are common problems on hydrophobic (HPB) and hydrophilic (HPL) surfaces. Herein, by regulation of surface wettability, we report a controllable ultrafast impact superspreading behavior (superspreading time of ∼3.0 ms) without splash and retraction on superamphiphilic (SAPL) silicon surfaces. Analysis of dynamic wetting processes combined with observation of lateral force microscopy images on SAPL surfaces reveals the existence of a precursor film at the spreading edge induced by heterogeneous surface wettability at nanoscale. Further study indicates that the inhibition of splash results from the high liquid flux in precursor film, which suppresses the interposition of air at the spreading edge. The reduction of Laplace forces owing to the presence of precursor film inhibits retraction at the spreading frontier. Taking advantage of this impact superspreading behavior on SAPL surfaces, effective heat dissipation is demonstrated, offering uniform and high heat flux for the spray cooling process.
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